The Hallicrafters S-38D was the first shortwave radio I worked with. On it I heard everything around the world through a little half-rhombic and random-wire antenna.
Some notes in 2003-2004:
The oldest members of this series are fifty years old and many have slept for a few decades in attics around the world, (but mostly in the USA, with some in Japan). A descendent of 1930s radio design, the S-38D radios are extremely sturdy examples of 1950s electronics. Even with thirty to fifty year-old tubes, resistors, capacitors, and switches, the three radios I bought still work and within their capacities rival the sensitivity of new radios. They are clearly very simple designs however, and have some operating quirks characteristic of electron-tube radios which don't occur (or occur as readily) with their transistorized and uP descendents.
The Good:
The radio is very sensitive, tunes cleanly, has a good, rich tone to the speaker characteristic of tube sets (However, the speaker is a 4" paper cone model producing about 70dB at 30cm (12 in.).) The radio has a provision for a two-prong phone connector, characteristic of pre-coaxial and pre-RCA jack audio connectors. The radio runs off of either AC or DC, running from 105 to 125VAC or VDC into the power plug. ( I tested with ten 12V batteries in series, and while connecting 120V DC requires the usual respect for lots of VA, the radio cheerfully runs on DC.)
Also the radios have adjustable coils and capacitors, making them relatively easy to align (However, each of the four bands needs to be aligned individually). In testing with a variety of antennas, the S-38D seems to receive proportionally to antenna length/area. The bigger the antenna the better it receives. In AM broadcast testing, it was very selective and never overloaded. The tuning also comes with a simple 0 to 100 logscale bandspread, which offsets the tuned frequency by a few kilocycles.
Also, the radio has a receive/standby mode switch. When selected to "STANDBY", the IF amplifier is disabled. In the "RECEIVE" setting, the IF amp is powered by the receive/standby mode switch.
The radio receives double-sideband standard AM when the MODE switch is in AM, but goes to single-sideband (LSB) when in CW. SSB is tuned by frequency offset. There is no BFO function.
The Interesting:
One interesting point about the design of the S-38D (and the 5R10A from which the S-38D is an extremely close, nearly identical, cousin) is that it doesn't propagate static with the same amplitude as most transistorized units. When there is no signal, the S-38D very quietly hisses. When tuning onto a signal, the signal comes clearly and loudly through the speaker (loudness is proportional to the "VOLUME" knob). From a distance, the detector quiets down AF output in effect (but theoretically different) like a modern squelch when a station ends transmission or the radio is tuned off-frequency.
Also, if the antenna lead is grounded to earth, and the radio is being powered from 120VAC, the speaker will hum like an ungrounded phono connector on a typical hi-fi stereo system. (On 120VDC power, the radio never hums at all.)
The bandspread control is a simple 0 to 100 logscale which negatively offsets the radio's frequency by a variable and unknown level as the bandspread tends from 0 to 100. (That is, as the bandspread value increases on the logscale, the tuned frequency of the radio decreases, but not by a definite quantity).
Like any tube set, the radio's tubes produce heat, with the two tubes under the speaker's rear half near the left rear of the radio producing the most heat. The left rear of the radio case is the warmest part of the case, but is merely warm. One could rest their hand on that part of the case forever without any discomfort (and might feel good if your hands are cold!). The sides and back of the case are about 5 or so degrees F above room temperature and the face and bottom of the set is at room temperature.
Also, typical of tube sets, the radio is inoperative for about the first fifteen seconds after power up as the thermoinic heaters in the tubes rise to their operating temperatures.
The display sidelight is driven by the voltage difference from the radio's tube regulator through two resistors. Changes in line voltage directly affect the brightness of the light, and during power-up the changes in heater current and electrical flow through the circuit cause the bulb to glow softly, then dim greatly to an orange filiment, then brighten steadily as the tubes warm up.
The radio has no printed circuit boards. The electrical components are directly soldered to the lugs on the tube sockets, to the switches, or to each other. It looks a little like a rats nest of components compared to the relatively highly ordered PC boards, but it works fine.
The Bad:
For what it does, the S-38D is pretty good, but there are a few annoyances even from a 1930s through 1950s perspective. First, the radio has a tendency to image on strong signals. Particularly noticeable in CW mode, the radio will quietly heterodyne more signals than are actually there. However, this is minor and images are weak compared to on-frequency signals.
Also, on the 50-year old models, the masonite-style rear and underside of the radio are understandably slightly bowed from gravity. Also, the switch selecting between the speaker and the phones is on the back of the unit near the phones connector (but this is only a minor issue). Also, the lighting on the vernier frequency display is on the extreme left of the verner, overlighting the first 1/8th of the left side while insufficiently illuminating the center through right side of the vernier at night.
The placement of the bulb has a traditional bias which carries back to the 5R10A, which had a blackfaced vernier display better suited to the sidelighting.
The radio has very good selectivity, but being a vernier dial, one doesn't exactly know what frequency the radio is actually on +/- about 2 kHz. Also, the radio drifts typically when cold, and when the temperature of the radio's components change. In still air after a few hours to warm the case and chassis the radio stabilizes.
The sidelight is affected by inrush current and changes in line voltage. If the radio is turned off then back on quickly, the sidelight will become extremely bright for about half a second, then gradually dim down to it's normal glow. Too many cycles like that will burn out the bulb's filament.
Conclusion:
The radio's got a lot of quirks typical of the radios of the 1950s and is a very simple and basic radio, but it receives basic AM and CW as well as any modern receiver (the EM physics of AM radio haven't changed). It's a very simple, unassuming, direct, likable little radio. |